Optical probes are essential to many lab-on-a-chip systems which require precise time-resolved information on the refractive index of a fluid medium. Considerable effort, therefore, has been focused on developing techniques to measure the refractive index of picoliter samples using refractive index probes. One approach is to integrate a refractometer into the microfluidic device, with examples including instruments based on beam deflection photonic crystal resonators Fabry-Perot interferometers and other microfabricated resonators. The best of these methods and systems can achieve a resolution of 10−7 refractive index units (RIU). The principal disadvantage is the need to microfabricate the refractometer and integrate it with the microfluidic system.
Chemically synthesized colloidal spheres dispersed in the fluid medium also have been used as probes of the local refractive index through spectroscopy of their whispering gallery modes. When these modes are excited by broadband fluorescence of dye molecules or quantum dots embedded in the probe sphere itself, the local refractive index can be measured with a resolution of 2.5×10−4 RIU. The necessary spectroscopic measurements, however, limit this approach to addressing one probe particle at a time. The fluid medium, moreover, must be chemically and physically compatible with the specially synthesized spheres.